Our research is mainly devoted to ultrafast dynamics in condensed matter. In particular, we focus on terahertz (THz) processes, their fundamental physics and how such dynamics can be applied in devices for future information technology.
The terahertz (THz) frequency band is part of the electromagnetic spectrum and extends from about 100 GHz to 100 THz. Although many important electronic and molecular properties become visible in this band, it was hard to reach for a long time because of the lack of sensitive techniques. Today, so called coherent THz techniques give access to this band. They distinguish from most other optical techniques by the fact that the electric field of the THz-signals is directly detected in the time-domain. In contrast, in the optical part of the spectrum only time-averaged light intensities can be detected. The time-resolved detection of the electromagnetic fields is advantageous, because amplitude and phase information become directly accessible. The analysis of THz-data allows to derive a complete picture of the dielectric properties of materials or even entire devices.
A typical THz pulse as recorded in our lab
The key to THz spectroscopy is the generation of ultrashort electromagnetic pulses. Coherent far-infrared light can be generated for instance by femtosecond laser excitation of semiconductors or antenna structures. Outstanding property of THz-technologies is that the temporal evolution of the field of the THz signal can be measured.
Semiconductor heterostructures are ideal for studies on electronic dynamics, which are stimulated by a light wave. In these structures quantized electronic states can be realized by modern semiconductor growth techniques. Illumination with THz pulses excites transitions between intersubband states. In our experiments, the electronic dynamics are mapped in time-domain by time-resolved THz spectroscopy. Besides their relevance for the understanding of fundamental physics, THz intersubband transitions have potential for ultrafast applications in communication technology.
Excitation of an electronic intersubband transition by absorption of a THz pulse
Excitation of an intersubband transition by absorption of a THz pulse
Right side: Excitation of an intersubband transition by absorption of a THz pulse. In this experiment the frequency of the THz pulse is strongly detuned from the frequency of the transition. During the presence of the THz pulse the electrons follow the oscillation of the driving field. After the decay of the driving pulse, the oscillatory 'motion' of the electrons relaxes into an oscillation at their eigenfrequency, which is higher than the frequency of the THz pulse. The lower graph shows a calculation of the population of the upper state.
Schematic of our apertureless THz microscope
Recently, we developed a THz microscope, which allows for spatial resolutions of the order of 100 nm. We now apply the microscope for tracking the ultrafast dynamics of electrons in semiconductor structures. The two goals of this research are:
